1. Field of the Invention
The present disclosure relates to an image sensor of very small size. More specifically, the present invention relates to an image sensor in which the pixels are separated by deep insulating walls.
2. Discussion of the Related Art
FIG. 1 schematically illustrates an example of a circuit of a cell of an array of photosensitive cells of an image sensor. With each photosensitive cell of the array are associated a precharge device and a read device. The precharge device is formed of an N-channel MOS transistor M1, interposed between a supply rail Vdd and a read node S. Precharge transistor gate M1 is capable of receiving a precharge control signal RST. The read device is formed of the series connection of first and second N-channel MOS transistors M2 and M3. The drain of first read transistor M2 is connected to supply rail Vdd. The source of second read transistor M3 is connected to an input terminal P of a processing circuit (not shown). The gate of first read transistor M2 is connected to read node S. The gate of second read transistor M3 is capable of receiving a read signal RD. The photosensitive cell comprises a photodiode D having its anode connected to a source of a reference voltage GND, for example, the circuit ground, and having its cathode connected to node S via an N-channel charge transfer MOS transistor M4. The gate of transfer transistor M4 is capable of receiving a charge transfer control signal T. Generally, signals RD, RST, and T are provided by control circuits not shown in FIG. 1 and may be provided to all the photosensitive cells of a same row of the cell array. Node S plays the role of a charge storage region.
The operation of this circuit will now be described. A photodetection cycle starts with a precharge phase during which a reference voltage level is imposed at read node S. This precharge is performed by turning on precharge transistor M1. Once the precharge has been performed, precharge transistor M1 is turned off. The reference charge status at node S is then read. The cycle carries on with a transfer to node S of the photogenerated charges, that is, the charges created and stored, in the presence of a radiation, in photodiode D. This transfer is performed by turning on transfer transistor M4. Once the transfer has been performed, transistor M4 is off and photodiode D once again starts photogenerating and storing charges which will be subsequently transferred to node S. Simultaneously, at the end of the transfer, the new charge status at node S is read. The output signal transmitted to terminal P then depends on the pinch of the channel of first read transistor M2, which is a direct function of the charge stored in the photodiode.
FIG. 2 shows a simplified top view of an image sensor and a conventional example of distribution of the electronic components (photodiodes and transistors) associated with the image sensor. The transistors and the photodiodes associated with the photosensitive cells are generally formed at the center of the image sensor at the level of block 11 (pixels). All around block 11 are formed the transistors of the peripheral transistors which, generally, carry out several processings of the signals associated with the photosensitive cells. As an example, blocks 13 (readout) correspond to the circuits dedicated to the provision of the control signals of the array of photosensitive cells and to the reading of the signals provided by the photosensitive cells. Generally, other peripheral circuits may be provided to perform additional functions directly at the level of the image sensor such as, for example, the correction of defects of the signals read from the read nodes of the photosensitive cells, the storage of images, signal processing operations, etc. Thus, block 15 (memory) may correspond to peripheral circuits dedicated to the storage of images. Blocks 17 (digital) may correspond to peripheral circuits dedicated to the performing of signal processing operations. Blocks 19 (IOs) may correspond to peripheral circuits dedicated to the processing of input/output interface signals and especially comprise transistors which are directly connected to the connection pads of the image sensor.
Usually, photosensitive cells forming image sensors are formed in and on a silicon substrate of crystal orientation (100). To avoid interference or crosstalk phenomena between pixels, insulating walls are formed between them. Several techniques are known to form such walls. For example, an upper portion of the substrate may be etched, and the obtained openings may be filled with an insulating material, for example, silicon oxide (STI technique).
Another technique comprises forming insulating walls in the form of heavily-doped regions. For example, if the substrate is of type P, heavily-doped P-type walls are formed around the pixel elements. Thus, the electrons of electron/hole pairs created close to these walls are pushed back towards the capture area of the photodiode and are not captured by a neighboring photodiode.
Such insulating walls are generally formed by implantation of dopant atoms from the substrate surface. The implantation system is placed along a direction forming a non-zero angle with the direction perpendicular to the substrate surface, generally ranging between 6 and 8°. This enables to become independent from possible variations of the implantation angle of the implantation systems, such variations being currently approximately 1°. Indeed, when the angle varies by 1° around an angle of approximately 7°, the implantation depth varies little for a same implantation power.
If the implantation direction approaches the direction perpendicular to the substrate surface, the implantation depth may vary significantly. This can be explained by the crystal structure of the substrate: the atoms of a silicon substrate of crystal orientation (100) form, in the direction perpendicular to the substrate surface, channels in which dopant atoms penetrate more easily than in slightly oblique directions. This effect is called “channeling”.
The current tendency to decrease the size of electronic and opto-electronic components has led designers to form circuits comprising elements having lateral dimensions, in the plane of the substrate surface, which are increasingly small. This decrease generally goes along with a decrease in the depth of the elements, perpendicularly to the substrate surface. In the case of image sensors, the lateral dimensions are desired to be decreased, but a relatively large depth should be maintained, typically from 2 to 3 μm for image sensors intended to receive light rays in the visible spectrum. Indeed, when silicon is illuminated, the absorption depth of the photons depends on the wavelength of the light rays. For example, for red light rays, the absorption depth of photons is on the order of from 3 μm to 5 μm. Thus, the width of the pixels and of the insulating walls is desired to be decreased while maintaining deep walls to properly separate absorption areas corresponding to different pixels and limit the crosstalk.
Currently, to form insulating walls having a depth of approximately 3 μm, implantations of dopants at high powers, for example, ranging between 500 and 1,000 keV, are performed. To perform such implantations, it is necessary to form a resin mask of significant thickness in front of the substrate. Typically, for insulating walls having a 3-μm depth, the resin mask has a thickness ranging between 2 and 2.5 μm. The significant thickness of the resin mask does not enable forming, therein, openings of small size. Thus, the use of such a mask limits the width of the insulating walls which may be formed. Further, since the dopant implantations are performed in a direction forming a non-zero angle with the direction perpendicular to the substrate, the thick mask tends to hinder and to limit the implantation (shadowing effect). Thus, with current techniques, the width of the insulating walls cannot be smaller than approximately 0.7 μm for a depth of approximately 3 μm.
Pixels having lateral dimensions on the order of 1.4 μm when the insulating walls have a 0.7-μm depth are currently formed. The remaining surface for each active area of the pixels then only amounts to one quarter of the total surface area of the pixels. If the lateral dimensions of the pixels are decreased without modifying the width of the insulating walls, this proportion is further decreased.
Thus, current techniques do not enable to design image sensors formed of pixels of small size in which the insulating walls have a very large shape factor (a depth of more than 2 μm for a width of less than 0.7 μm).